The effect of temperature on the void nucleation and growth is studied using the molecular dynamics (MD) code LAMMPS (Large-Scale Atomic/Molecular Massively Parallel Simulator). Single crystal copper is triaxially expanded at 5 × 109 s−1 strain rate keeping the temperature constant. It is shown that the nucleation and growth of voids at these atomistic scales follows a macroscopic nucleation and growth (NAG) model. As the temperature increases there is a steady decrease in the nucleation and growth thresholds. As the melting point of copper is approached, a double-dip in the pressure–time profile is observed. Analysis of this double-dip shows that the first minimum corresponds to the disappearance of the long-range order due to the creation of stacking faults and the system no longer has a FCC structure. There is no nucleation of voids at this juncture. The second minimum corresponds to the nucleation and incipient growth of voids. We present the sensitivity of NAG parameters to temperature and the analysis of double-dip in the pressure–time profile for single crystal copper at 1250 K.
High velocity impact of copper plates using molecular dynamics has been performed to study the spallation of single crystal copper at impact velocities of 1100 and 1000 m s−1. The molecular dynamics code LAMMPS (Large-Scale Atomic/Molecular Massively Parallel Simulator) with the embedded atom method potential is used for this study. It is found that for an impact velocity of 1100 m s−1, nucleation and growth of multiple voids take place which lead to the spallation of the material. For the impact at 1000 m s−1 in the ⟨1 0 0⟩ impact direction, the material does not undergo spallation but gives a spall-like signal in the free surface velocity of the target. We show that the tension developed by first traversal of the shock wave creates various kinds of defects in the target. These become void nucleation sites during the subsequent traversal of the shock wave. The presence of void nucleation sites due to the first traversal of the shock leads to the nucleation of the voids at a lower tensile pressure. We also show that the spall-like signal in the free surface velocity of the target at 1000 m s−1 impact along the ⟨1 0 0⟩ direction occurs due to stress relaxation resulting from the nucleation and growth of the voids without physical separation of scab from the target.
Quasi-static (0.0033 s À1 ) and dynamic (10 3 s À1 ) compression experiments were performed on single crystal copper along h100i and h110i directions and best-fit parameters for the Johnson-Cook (JC) material model, which is an important input to hydrodynamic simulations for shock induced fracture, have been obtained. The deformation of single crystal copper along the h110i direction showed high yield strength, more strain hardening, and less strain rate sensitivity as compared to the h100i direction. Although the JC model at the macro-scale is easy to apply and describes a general response of material deformation, it lacks physical mechanisms that describe the influence of texture and initial orientation on the material response. Hence, a crystal plasticity model based on the theory of thermally activated motion of dislocations was used at the meso-scale, in which the evolution equations permit one to study and quantify the influence of initial orientation on the material response. Hardening parameters of the crystal plasticity model show less strain rate sensitivity along the h110i orientation as compared to the h100i orientation, as also shown by the JC model. Since the deformation process is inherently multiscale in nature, the shape changes observed in the experiments due to loading along h100i and h110i directions are also validated by molecular dynamics simulations at the nano-scale. V C 2014 AIP Publishing LLC.
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